What is a geomagnetic storm?

After a brief review of magnetospheric and interplanetary phenomena for intervals with enhanced solar wind-magnetosphere interaction, an attempt is made to define a geomagnetic storm as an interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of the quantifying storm time Dst index. The associated storm/substorm relationship problem is also reviewed. Although the physics of this relationship does not seem to be fully understood at this time, basic and fairly well established mechanisms of this relationship are presented and discussed. Finally, toward the advancement of geomagnetic storm research, some recommendations are given concerning future improvements in monitoring existing geomagnetic indices as well as the solar wind near Earth.